commutator

ABSTRACT

A commutator  10  comprises an electrically non-conductive base  12;  and a plurality of electrically conducting segments  16  supported by the base  12.  Each segment  16  comprising a first electrically conductive inner layer  18  provided on the base  12,  and a second electrically conductive outer layer  20  fixed to the inner layer  18.  The inner and outer layers  18, 20  have opposing surfaces  21, 22  which abut each other, once the commutator is assembled. An opening, preferably being a recess  30,  for receiving flux and/or air during the fixing of the outer layer to the inner layer is provided in at least one of the opposing surfaces  21, 22.  There is also provided a DC electric motor which includes such a commutator  10.

CROSS REFERENCE TO RELATED APPLICATIONS

This non-provisional patent application claims priority under 35 U.S.C.§119(a) from Patent Application No. 0800464.0 filed in United Kingdom onJan. 11, 2008.

FIELD OF THE INVENTION

The present invention relates to a commutator for an electric machineand a method of improving a connection between a first electricallyconductive inner layer of a commutator segment and a second electricallyconductive outer layer fixed to an outer surface of the inner layer.

BACKGROUND OF THE INVENTION

Commutators, being rotary switches used with DC electric motors,comprise multiple electrically conductive segments arranged into acylinder or plane and anchored into a non-conducting, typically phenolicresin, moulding compound. Each segment is physically separated andelectrically isolated from those adjacent to it, so that an electrical,typically carbon, brush passing along the outer surface thereof willform a conductive path only with the segment (or segments) in contactwith it at any given instant.

FIGS. 1 to 4 show one known planar commutator 1. This commutatorcomprises a circular phenolic electrically non-conductive base 2 havinga central aperture 3 for a motor shaft, and a plurality of electricallyconductive segments 4 supported by the base 2. Each segment 4 includes acopper inner layer 5 from which a tang 6 extends for connection to awinding of the rotor, and a graphite brush-contacting outer layer 7which is fixed to the copper inner layer 5.

The inner layer 5 of the segment is formed with a number of barbs 8, inthis case being three. The barbs project at a slight angle from edges ofthe inner layer, and locate at edges of the electrically non-conductivebase 2. While this arrangement does provide a particularly secure androbust attachment, it prevents the commutator from being miniaturiseddue to the process of forming the radially inner barb. This poses a sizelimitation related to the number of segments. Omitting the inner barbleads to an unstable connection between the segment and the base which,during use in harsh vibrational environments, can result in loosening.

To attach the graphite outer layer 7 of the segment to the copper innerlayer 5, soldering is typically used. However, using X-ray imaging, ithas been found that excess flux and air is trapped between the twolayers 5, 7 during attachment, thus resulting in a non-uniform andweaker connection.

With the copper inner layer 5 connected to the phenolic base 2 and thegraphite brush-contacting outer layer 7 connected to the inner layer 5,the segments of the commutator are then formed by cutting, typicallywith a circular saw being drawn diagonally across the commutator.However, it has been found that the outer perimeter edge of the phenolicbase 2 is often chipped by the cutting device as the cutting device isdrawn out and away from the base. This results in a higher scrap rate.The chipped area weakens the commutator base, and thus cracking can moreeasily occur, accelerating brush wear and impacting the longevity of themotor. Also, the chipped area may prevent a mould from fully sealingaround the perimeter edge, thus allowing armature overmould plastic toenter into the commutator slot, which shortens the life of thecommutator.

SUMMARY OF THE INVENTION

Embodiments of the present invention seek to overcome one or more of theabove-mentioned problems.

According to a first aspect of the present invention, there is provideda commutator comprising an electrically non-conductive base; and aplurality of electrically conducting segments supported by the base,each segment comprising a first electrically conductive inner layerprovided on the base, and a second electrically conductive outer layerfixed to the inner layer, the inner and outer layers having opposingsurfaces which abut each other, characterised in that an opening forreceiving flux and/or air during the fixing of the outer layer to theinner layer is provided in at least one of the opposing surfaces.

Preferably, the said opening is a bottomed recess.

Advantageously, the opening may be provided in the opposing outersurface of the inner layer.

Optionally, the opening may be provided in the opposing inner surface ofthe outer layer.

Preferably, the opening is formed by pressing.

Advantageously, the commutator may further comprise an anchor whichanchors the inner layer to the base. In this case, the anchor mayoptionally be formed from a portion of the inner layer.

Preferably, the anchor is formed from material pressed from the innerlayer when forming the said opening.

Furthermore, the anchor may preferably have an inverted V-shape.

Beneficially, the anchor may be spaced from an edge of the inner layer.

Preferably, the inner layer of each segment includes a second opening inwhich is received a portion of the base. In this case, the secondopening may preferably be an aperture which extends through the innerlayer.

Preferably, the segment includes a tang formed from the inner layer.

Preferably, the inner layer is metal, the outer layer is graphite, andthe base is a resin.

Advantageously, the electrically non-conductive base may preferablyinclude a plurality of pre-formed notches for preventing or limitingchipping during segmentation.

Beneficially, the commutator may be planar. Furthermore, there mayoptionally be sixteen electrically conducting segments.

According to a second aspect of the present invention, there is provideda DC electric motor comprising a motor housing, stator in the housing, arotor having a shaft rotatably mounted in the housing, a rotor corefixedly provided on the shaft and juxtaposed to the stator, a commutatorin accordance with the first aspect of the invention and which isfixedly mounted on the shaft, and brush gear in electrical contact withthe commutator.

According to a third aspect of the invention, there is provided a methodof improving a connection between a first electrically conductive innerlayer of a commutator segment and a second electrically conductive outerlayer fixed together by abutting opposing surfaces, the methodcomprising the step of forming an opening in at least one of theopposing surfaces prior to fixing together the layers, the openingaccommodating flux and/or air trapped between the inner and outer layersduring fixing, so that a more uniform connection between the layers isachieved.

Preferably, the opening is a recess which is formed in the inner layerby pressing.

The method preferably further comprises a second step subsequent to thefirst said step of soldering the inner and outer layers together.

Advantageously, the method may optionally further comprise a third stepsubsequent to the first said step of forming an anchor on the innerlayer, the anchor being formed from material pressed from the innerlayer when forming the said opening.

Furthermore, the method may further comprise an optional fourth stepsubsequent to the first said step of fixing the inner layer to anelectrically non-conductive base. In this case, an insert mouldingprocess may preferably be used to connect the base and the inner layer.

Preferably, the method further comprises a fifth step during orsubsequent to the fourth step of forming a plurality of notches in thesaid base to prevent or limit chipping during cutting. In this case, themethod further comprises an optional sixth step subsequent to the fifthstep of cutting the inner and outer layers to form commutator segments,the or each cut being aligned with and passing through respectivenotches.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described, by way ofexample only, with reference to figures of the accompanying drawings. Inthe figures, identical structures, elements or parts that appear in morethan one figure are generally labelled with a same reference numeral inall the figures in which they appear. Dimensions of components andfeatures shown in the figures are generally chosen for convenience andclarity of presentation and are not necessarily shown to scale. Thefigures are listed below.

FIG. 1 shows a perspective view from a brush-contacting layer of a priorart segment or bar planar commutator once assembled and cut;

FIG. 2 shows a perspective view of the prior art commutator from below,the electrically non-conductive base being shown in phantom so that anelectrically conductive inner layer can be seen;

FIG. 3 shows a part of one of the prior art segments which forms theinner layer and to which the brush-contacting layer is fixed;

FIG. 4 shows an enlarged portion of the edge of the prior artcommutator, prior to segmentation;

FIG. 5 shows a perspective view of one embodiment of a commutator, inaccordance with the first aspect of the invention, with the outerbrush-contacting layer removed for clarity and prior to segmentation;

FIG. 6 shows the outer brush-contacting layer of the commutator shown inFIG. 5, prior to attachment to the electrically conductive inner layer;

FIG. 7 is a cross-sectional view through one segment of the commutatorof FIG. 5, showing the anchoring of the inner layer to thenon-conductive base and the outer brush-contacting layer fixed to theinner layer;

FIG. 8 is a diagrammatic cross-sectional view showing a principle ofattachment of the outer brush-contacting layer to the inner layer;

FIG. 9 is a perspective view of the inner layer of one segment of thecommutator, showing the formation of a recess;

FIG. 10 is a view similar to FIG. 9 of the segment showing the formationof the anchor; and

FIG. 11 is an enlarged portion of the edge of the commutator shown inFIG. 5, showing a notch in the base.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring firstly to FIGS. 5 to 11, there is shown a commutator 10 whichcomprises an electrically non-conductive disk-like base 12 having acentral aperture 14 for receiving a motor shaft, and a plurality ofelectrically conducting segments 16 supported by the base 12. The base12 of the commutator 10 is typically formed from phenolic mouldingcompound, but any suitable electrically non-conductive material can beused.

Each segment 16 comprises a first electrically conductive inner layer 18which is attached to the base 12, and a second electrically conductivebrush-contacting outer layer 20 fixed via its inner surface 21 to anopposing outer surface 22 of the inner layer 18. The inner layer 18 ispreferably metal, for example copper or copper over an aluminium core.The outer layer 20 is preferably graphite. Both inner and outer layers18, 20 also include a central aperture 24 for accommodating a motorshaft, and the inner layer 18 includes a tang 26 extending from itsouter perimeter edge for connection to a motor winding.

The outer surface 22 of the inner layer 18 of each segment 16, beingopposite the base-contacting surface 28, includes a recess 30. Eachsegment 16 includes at least one of the recesses 30 formed partwaybetween the inner edge 32 of the inner layer 18 and the outer edge 34.The recess 30 is bottomed, and is preferably formed by pressing, as bestunderstood from FIG. 9. Although the recess 30 is circular, anynon-circular shape can also be used.

FIGS. 5 to 7 show the principle behind the use of the recess 30. Duringassembly of the commutator 10 and prior to segmentation, theelectrically conductive disk-like inner layer 18 with a plurality ofrecesses 30 corresponding to the number of segments 16 is attached tothe base 12. The inner surface 21 of the disk-like brush-contactingouter layer 20 is then fixed to the outer surface 22 of the inner layer18. Fixing is typically by soldering, and the recesses 30 provide aspace to which excess solder flux and air trapped between the abuttingsurfaces can move, as diagrammatically shown in FIG. 8. It isascertained by X-ray imaging that, by providing the recesses 30, theattachment between the inner and outer layers 18, 20 is significantlymore uniform, resulting in a stronger and longer lasting connection.Once assembled, the inner and outer layers 18, 20 are segmented to formthe segments 16 by a cutting device passing diametrically thereacross.

The formation of the recess 30 by pressing results in a portion 36 ofthe inner layer 18 projecting from the base-contacting surface 28, asshown in FIG. 9. By splitting this portion 36 into two arms 38 (see FIG.10), thus forming an inverted V-shape, an anchor 40 is formed. Theanchor 40 is spaced from the edges of the inner layer 18 of each segment16. Preferably, the anchor 40 is substantially central of the innerlayer 18, and preferably matches the position of the respective recess30.

The inner layer 18 of each segment 16 also includes an opening 42 at oradjacent to its inner edge 32. The opening 42, in this embodiment, is afixing aperture which extends through the thickness of the inner layer18. However, the opening 42 could be a bottomed recess formed in thebase-contacting surface 28 or a slot in the inner edge 32.

During assembly of the commutator 10, the inner layer 18 is mounted onthe uncured base 12 so that the anchor 40 is embedded in the mouldingcompound and so that the moulding compound passes into and through thefixing aperture 42. This is best done by moulding the base 12 directlyto the inner layer 18 for example by using an insert moulding techniquein which the inner layer 18 is inserted into the die of a plasticmoulding machine before plastics material is injected into the die tomould the base 12. Once cured, the inner layer 18 is securely andnon-releasably held to the base 12 at a position partway between theinner and outer edges 46, 48 of the base 12 via the anchor 40, and alsoby the inner edge 32 of the inner layer 18 being encased in the base 12.This encasement forms a ring which is used to position the outer layer.

The centralised anchor 40 provides an extremely positive attachment ofthe inner layer 18 to the base 12, and the fixing aperture 42 andembedding of the inner edge 32 of the inner layer 18 reinforces theattachment.

Once the electrically non-conductive base 12, and electricallyconductive inner and outer layers 18, 20 have been connected together asdescribed above, the required number of segments 16 are formed bycutting diagonally there across using, for example, a circular saw orcutting disc. In this embodiment, sixteen segments are formed, andtherefore eight cuts are made. However, the embodiment of the commutatordescribed so far is applicable to any number of segments and is notlimited to just sixteen.

Since the moulding compound of the base 12, once cured, is relativelybrittle, notches 50 are formed in the perimeter surface 52 midwaybetween the tangs 26, during moulding and prior to cutting orsegmentation. The notches 50 are best seen in FIG. 11. The notch 50 is arecess, depression, slot, channel or slit formed in the outer edge 48aof the base 12 nearest to the inner layer 18 of the segments 16. Thecuts are aligned with the location of the notches. Consequently, as thecutting device is moved across the diameter of the inner and outerlayers 18, 20 during the segmentation process, the cutting blade such asa circular saw or cutting disc, first passes through a notch beforestarting to cut the base and at the end of the cutting stoke, the bladepasses through another notch. The notches 50 prevent or limit chippingof the base 12 by giving physical support to the area of the base beingcut.

The use of the notches 50 described above is applicable to othercommutators assembled in different manners, providing segmentation isformed by cutting.

The above described recess(es) in the outer layer of the inner surfaceis an essential feature. However, the anchor, the opening at or adjacentto the inner edge of the inner layer, and the notches are preferablefeatures.

Although a plurality of discrete recesses are suggested, a single recesscan be utilised. For example, an endless circular-shaped recess, whichmay be a slot or channel, could be used, which, following segmentation,provides a discrete slot or channel across each segment.

Any suitable means for connecting the inner and outer layers can beutilised, including soldering, welding and brazing. However, bonding isnot precluded. In all of these cases, the recess may accept flux, butmay alternatively or additionally accept air or any other material tothus provide a more uniform engagement between the two layers.

Although the use of a recess is described above, any suitable openingcan be utilised. For example, the opening may be a through-hole whichextends fully through the inner layer.

Although the recess or opening is described as being provided in theinner layer, the recess or opening, in addition to or as an alternative,can be provided in the opposing inner surface of the outer layer. Inthis case, the opposing surfaces of the inner and outer layers whichabut each other may both each have an opening for receiving flux and/orair during the fixing of the outer layer to the inner layer.Alternatively, the opening may only be provided in one of the opposinglayers.

The above described commutator is beneficial for use in a DC motor, andin particular 12 volt and 24 volt DC electric motors. The motor hasmainly standard components, and in particular a motor housing, a statorhoused in the motor housing, and a rotor having a shaft rotatablymounted in the housing, a rotor core fixedly provided on the shaft to bejuxtaposed to the stator, and a commutator. Brush gear is also providedin or on the housing to be in electrical contact with the commutator.

The benefits of being able to utilise the commutator described above fora DC electric motor are that electrical resistance is lowered due tobetter connection between the outer and inner layers, more robustphysical connection between the inner layer and the base in a smallerpackage, and a longer working life is achieved. A high rated 24 volt DCelectric motor allows a 24 volt fuel pump and other 24 volt motorsystems to be developed, thus allowing the development of the desired 24volt electrical system for vehicles without the need for voltagechanging devices. This not only reduces cost, but also saves weight andspace.

Typical applications for a 24 volt DC electric motor are within any 24volt system, such as found in trucks, tractors, and passenger vehicles.

The embodiments described above are provided by way of examples only,and various modifications will be readily apparent to the skilled personwithout departing form the scope of the invention as defined by theappended claims.

In the description and claims of the present application, each of theverbs “comprise”, “include”, “contain” and “have”, and variationsthereof, are used in an inclusive sense, to specify the presence of thestated item but not to exclude the presence of additional items.

1. A commutator comprising an electrically non-conductive base; and aplurality of electrically conducting segments supported by the base,each segment comprising a first electrically conductive inner layerprovided on the base, and a second electrically conductive outer layerfixed to the inner layer, the inner and outer layers having opposingsurfaces which abut each other, and an opening for receiving flux and/orair during the fixing of the outer layer to the inner layer provided inat least one of the opposing surfaces.
 2. The commutator of claim 1,wherein the opening is a bottomed recess.
 3. The commutator of claim 1,wherein the opening is provided in the opposing outer surface of theinner layer.
 4. The commutator of claim 1, wherein the opening isprovided in the opposing inner surface of the outer layer.
 5. Thecommutator of claim 1, wherein the opening is a pressed recess.
 6. Thecommutator of claim 1, further comprising an anchor which anchors theinner layer to the base.
 7. The commutator of claim 6, wherein theanchor is formed from a portion of the inner layer.
 8. The commutator ofclaim 6, wherein the anchor is formed from material pressed from theinner layer when forming the opening.
 9. The commutator of claim 6,wherein the anchor has an inverted V-shape.
 10. The commutator of claim6, wherein the anchor is spaced from an edge of the inner layer.
 11. Thecommutator of claim 1, wherein the inner layer of each segment includesa second opening in which is received a portion of the base.
 12. Thecommutator of claim 11, wherein the second opening is an aperture whichextends through the inner layer.
 13. The commutator of claim 1, whereinthe segment includes a tang formed from the inner layer.
 14. Thecommutator of claim 1, wherein the inner layer is metal, the outer layeris graphite, and the base is a resin.
 15. The commutator of claim 1,wherein the electrically non-conductive base includes a plurality ofpre-formed notches for preventing or limiting chipping duringsegmentation.
 16. The commutator of claim 1, wherein the commutator isplanar.
 17. The commutator of claim 1, wherein there are sixteenelectrically conducting segments.
 18. A DC electric motor comprising: amotor housing, stator in the housing, a rotor having a shaft rotatablymounted in the housing, a rotor core fixedly provided on the shaft andjuxtaposed to the stator, a commutator as defined in claim 1 fixedlymounted on the shaft, and brush gear in electrical contact with thecommutator.